The method and brazing tape described herein relates generally to thermal barrier coatings. More specifically, the method relates to forming micro channels in a thermal barrier coating with the use of a brazing tape or preform having water-soluble ceramic members.
In gas turbines, air is drawn into and is compressed by a shaft-mounted rotary-type compressor. The compressed air is mixed with fuel in combustors. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on a shaft. The flow of gas turns the turbine, which turns the shaft/rotor and drives the compressor and fan (for aircraft applications). In land based applications, the turbine may drive a generator. In aircraft applications, the hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
During operation of gas turbines, the temperatures of combustion gases may exceed 3,000° F., considerably higher than the melting temperatures of the metal parts of the turbine, which are in contact with these gases. Operation of these turbines at gas temperatures that are above the metal part melting temperatures is a well-established art, and depends in part on supplying cooling air to the metal parts through various methods. The metal parts that are particularly subject to high temperatures, and thus require particular attention with respect to cooling, are the metal parts forming combustors and parts located aft of the combustor.
The hotter the turbine inlet gases, the more efficient is the operation of the turbine. There is thus an incentive to raise the turbine inlet gas temperature. However, the maximum temperature of the turbine inlet gases is normally limited by the materials used to fabricate the components downstream of the combustors such as the vanes and the blades of the turbine. In current engines, the turbine vanes and blades are made of nickel-based superalloys, and can operate at temperatures of around 2,100° F.
The metal temperatures can be maintained below melting levels with current cooling techniques by using a combination of improved cooling designs and thermal barrier coatings (TBCs). For example, with regard to the metal blades and vanes employed in gas turbines, some cooling is achieved through convection by providing passages for flow of cooling air from the compressor internally within the blades so that heat may be removed from the metal structure of the blade by the cooling air. Such blades have intricate serpentine passageways within the structural metal forming the cooling circuits of the blade.
Small internal orifices have also been devised to direct this circulating cooling air directly against certain inner surfaces of the airfoil to obtain cooling of the inner surface by impingement of the cooling air against the surface, a process known as impingement cooling. In addition, an array of small holes extending from a hollow core through the blade shell can provide for bleeding cooling air through the blade shell to the outer surface where a film of such air can protect the blade from direct contact with the hot gases passing through the engine, a process known as film cooling.
In another approach, a thermal barrier coating (TBC) is applied to the turbine blade component, which forms an interface between the metallic component and the hot gases of combustion. The TBC includes a ceramic coating that is applied to the external surface of metal parts to impede the transfer of heat from hot combustion gases to the metal parts, thus insulating the component from the hot combustion gas. This permits the combustion gas to be hotter than would otherwise be possible with the particular material and fabrication process of the component.
TBCs include well-known ceramic materials, for example, yttrium-stabilized zirconia (YSZ). Ceramic TBCs usually do not adhere well directly to the superalloys used as substrate materials. Therefore, an additional metallic layer called a bond coat is placed between the substrate and the TBC. The bond coat may be made of a nickel-containing overlay alloy, such as a MCrAlY, where M is an element selected from the group consisting of Ni, Co, Fe and combinations thereof, or other compositions more resistant to environmental damage than the substrate. Alternatively, the bond coat may be a diffusion nickel aluminide or platinum aluminide, which is grown into the surface of the substrate and whose surface oxidizes to form a protective aluminum oxide scale that provides improved adherence of the ceramic top coatings. The bond coat and overlying TBC are frequently referred to as a thermal barrier coating system.